The present invention relates to a ratio control system for a continuously variable transmission in a motor vehicle.
U.S. Pat. No. 4,735,113 discloses a continuously variable transmission for a vehicle. According to a known ratio control system for the continuously variable transmission, a feedback control is effected whereby a reduction ratio in the continuously variable transmission is controlled in such a direction as to decrease an input revolution speed error or deviation toward zero., That is, a target input revolution speed is determined by a table look-up operation of a shift point mapping based on vehicle speed and throttle opening degree. An error or deviation between the target input revolution speed and an actual input revolution speed is determined, and this deviation is used as a variable in a feedback control equation including a proportional term and an integral term. A result from calculating this equation is added to a target reduction ratio. An output signal indicative of this sum is supplied to a shift motor in the form of a stepper motor. This known feedback control poses a problem that, when the continuously variable transmission starts shifting up, the stepper motor does not turn by an angle large enough to compensate for a delay inherent with a hydraulic shift control circuit, causing a dull shift response. This derives from the fact that the feedback control is effected on the input revolution speed deviation only. In detail, at the beginning of shift where the vehicle speed and input revolution speed are low, the input revolution speed deviation is small. Under this condition, the feedback control value resulting from calculating the feedback control equation involving the input revolution speed deviation is small, As a result, an angle of rotation of the stepper motor induced is small, the displacement of a spool of a shift control valve is small, and a speed at which hydraulic fluid is supplied to a driver pulley is gradual. This provides a poor shift response and drivability.